Plasma immersion ion processing (PIIP) is a surface modification technique based on extracting accelerated ions from a plasma. A high voltage pulsed DC or pure DC power supply is used to target the ions to a surface. The process is applied to both ion deposition (PIID) and ion implantation (PIII).
The plasma can be generated in a suitably designed vacuum chamber and with various plasma sources. Examples of plasma sources are an electron cyclotron resonance plasma source, helicon plasma source, capacitively coupled plasma source, inductively coupled plasma source, DC glow discharge, metal vapor arc (for metallic species), and RF excitation. A pulsed DC glow discharge is commonly used and does not require an external source for plasma generation.
FIG. 1 illustrates an example of conventional plasma immersion deposition (PIID) processing, as used to coat the surface of three different workpieces. The workpieces are placed on a metal plate (worktable) in a vacuum chamber. Various apparatus that may be associated with external plasma sources and dopants are not explicitly shown, but are well known in the art of PIIP.
When a high negative voltage is applied to the worktable, at a pressure of a few tens of millitorr, plasma is generated around the workpieces and the worktable. The same voltage also draws positively charged ions from the plasma to the worktable and the surfaces of the workpieces. The impact of the incoming ions also generates electrons (secondary electrons) from the surfaces. These electrons travel to the chamber wall, which is positive with respect to the workpieces, to complete the circuitry.
At a high bias voltage, ions will be implanted into the surfaces. On the other hand, if the ion energy is low, a coating can be deposited on the surface. For example, if acetylene is used as a precursor, diamond-like carbon (DLC) coatings can be deposited.
Electron bombardment is another type of PIIP, but uses polarity opposite to that described above. The result is deposition or implantation of electrons rather than ions.
When the surface to be coated is dielectric, such as in the case of ceramic, glass or polymer materials, the PIII process can be particularly challenging. U.S. Pat. No. 5,374,456 teaches the use of a conductive grid placed conformally over the surface of the object. Other methods have involved placing the object on a metal plate so that ions are drawn to the nonconducting surface of the object as well as to the plate. However, these methods are useful when only one side of the object is to be coated.